There are known arms for spectacles made of composite material formed from a polymer matrix, generally epoxide-based, reinforced with fibres, typically of carbon.
Arms of this type, described for example in International Publication No. WO 2009/080444, are particularly valued not only for their distinctive appearance but also for their mechanical characteristics and the lightness imparted to them by the material from which they are made.
This material is formed by long fibers, present in the polymer matrix in a high percentage by weight of at least 50% and possibly as much as 70%, which can be processed so as to be substantially unidirectional, or so as to be intertwined or woven, embedded in a polymer matrix to form a layer of pre-impregnated material, also known as “pre-preg.”
It should be noted that these composite materials are clearly different, in terms of both their production and their mechanical characteristics, from composite materials comprising a polymer matrix in which reinforcing fibers of reduced length (known as “short fibers”) are embedded in proportions of up to 40% by weight, these fibers generally being dispersed in a random way in the matrix.
Arms of the aforementioned type are generally made by successively superimposing a suitable number of these layers, which are subjected, within a suitable mold, to temperature and pressure conditions which result in a process of consolidation and cross-linking of the composite material forming the superimposed layers. On completion of the cross-linking phase of the composite material, a sheet-like intermediate product is obtained, from which the arms are produced by appropriate cutting and edge trimming.
The resulting arms have substantially uniform mechanical properties along their longitudinal extension, particularly in terms of flexibility, elasticity and percentage elongation.
EP 2051128 describes an arm for spectacles made of composite material and capable of retaining over time the form imparted by manual deformation carried out to adjust the shape of the arm. The arm described in EP 2051128 has this advantageous characteristic because of the provision of a metallic core of constant thickness in the arm, surrounded by a coating of fiber-reinforced polymer-based composite material. The metallic core extends from a first longitudinal end of the arm, where it can act as a hinge element with the front frame, to the opposite longitudinal end. The arm described in EP 2051128 has a substantially constant thickness along the longitudinal extension of the arm, at least in its regions made of composite material.
The inventor has observed that, in some cases, spectacle arms should be provided with characteristics of flexural rigidity that are suitably differentiated along their longitudinal extension, in order to meet all the aesthetic and functional requirements of designers.
For example, in order to enable the spectacles to be worn comfortably and to be kept in a position adhering to the user's head, the arm preferably has regions of differentiated flexibility, with greater flexibility in the region near the hinge (so as to replace, possibly, the conventional but costly elastic hinges), followed by a greater flexural rigidity in the median region corresponding approximately to the temple of the user wearing the spectacles. Alternatively, or additionally, it may be preferable to have an arm with a more flexible region in the area above the ear and a more rigid adjacent region in the area behind the ear, allowing the spectacles to “grip” the user's head more effectively.
The inventor has also observed that this differentiation in the rigidity characteristics is obtained in spectacle arms made of other types of material, for example metal or polymer, by providing regions of increased thickness (thickening) in the regions where greater flexural rigidity is desired.
However, the specific composition and layered structure of the composite material used is unsuitable for creating variations of thickness.
This is because, in order to keep the mechanical and aesthetic characteristics of the composite material unaltered, the ratio between the polymer and the reinforcing fibers must be kept within a well-defined range in every portion of the arm. This characteristic makes it inherently impossible to form regions of greater thickness in a structure of simple superimposed layers by shaping the mold cavity in a suitable way (for example, by forming a recess with cavities of different depth). In fact, it has been observed that, in the first phase of molding, owing to the high temperature and the newly initiated cross-linking reaction, the polymer matrix becomes particularly fluid and tends to flow from regions of higher pressure towards regions of lower pressure. Consequently, if the mold impression has varying cross sections, the greater space available in the areas of larger cross section is substantially occupied by the polymer component, so that a composite material having a high percentage of polymer is produced in these regions, whereas, in the adjacent regions, a composite material with an excessively high percentage of fibers is obtained.
To avoid such kind of drawbacks, variations of thickness are usually created by superimposing successive portions of layers in the areas concerned, with progressively reduced sizes if necessary, so that, when placed in the mold, the various areas of the layered structure are substantially subjected to the same pressure and there is no significant displacement of polymer within the material.
This operation has the drawback of requiring a considerable labor load in terms of time and human resources. Moreover, in this type of arm the edge, which is formed by the trimmed edges of the layers forming the arm, does not have the typical aesthetic effect of the texture of composite carbon materials (known as the “carbon look”), which is visible only on the principal surfaces of the layers. Although this limitation is less significant in very limited thicknesses, it may negatively affect the arm at the regions of greater thickness and may be aesthetically displeasing.
It should be noted that, in the context of the present description and the subsequent claims, the term “rigidity” or “flexural rigidity” denotes the resistance of the arm or a region thereof to bending about an axis perpendicular to the longitudinal direction of the arm and substantially parallel to an axis about which the arm is hinged to a front frame of the spectacles to which the arm is intended to be fixed in a hinged arrangement. Similarly, in the context of the present description and the subsequent claims, the contrary term “flexibility” denotes the capacity of the arm or a region thereof to be deformed when subjected to bending of the same type.